1. Field of the Invention
The present invention relates to power systems. More specifically, the present invention is a system for the transfer of optical energy to a remote location and subsequent conversion of the transferred optical energy to another form of energy such as heat, electricity, or mechanical work.
2. Description of the Related Art
It has been known in the telecommunications industry for several decades that light, or optical energy, can be sent down a relatively small diameter (e.g., twenty-five micron) glass optical fiber, modulated, and used to send large amounts of low-noise data or voice channels in a manner superior to traditional metal conductors. The range (i.e., distance) over which an un-boosted signal can be sent down such a glass fiber is controlled by a number of physical phenomena along with the geometry of the fiber and the materials used in its construction.
As light travels down the fiber, a portion of the injected energy is lost due to several mechanisms including Rayleigh scattering, OH absorption, imperfection loss, and infrared absorption. First, Rayleigh scattering is a function of the wavelength of the injected laser light and of the fiber material (frequently silica glass). Aside from selecting a low impedance fiber, the only way to reduce Rayleigh scattering is to select the wavelength of light that produces the least power loss per unit length of fiber. Second, OH absorption loss can be controlled or reduced by constructing fibers with ultra low OH content and avoiding wavelengths that coincide with wavelength-specific OH loss peaks. Third, imperfection loss can only be reduced by use of a fiber with minimal or no manufacturing imperfections. Often this is an issue of quality control of raw materials and manufacturing processes that draw the fiber slowly so as to not introduce imperfections. Finally, infrared absorption loss is a function of wavelength and material. Aside from material selection and improvement infrared loss can only be minimized by choosing an optical frequency that minimizes the losses.
In addition to the optical transmission loss mechanisms just described, there are other potential power loss mechanisms including: thermal damage to fiber at very high temperatures (whether externally or internally produced); non-linear effects such as SRS (stimulated Raman scattering), and also “self-focusing.” Self-focusing has been predicted to potentially limit actual power delivery in a fiber to four or five megawatts regardless of fiber diameter, based on current theoretical assumptions and predictions. However, as with many previous theoretical predictions relating to estimates of power and power density limitations for lasers, these estimates may also prove overly conservative in time.
FIG. 1A shows a plot of the theoretical composite attenuation limits for optical power transmission as a function of wavelength per kilometer of pure silica fiber. Other materials and fiber constructions (e.g., hollow, mirror-coated fiber) will have different attenuation characteristics. However, currently, pure silica fiber is the most readily available material to work with and obtain in long lengths (on the order of tens of kilometers). The composite attenuation is at a minimum at approximately 1540 to 1550 nanometers (nm) wavelength. This is due mainly because Rayleigh scattering decreases with increasing wavelength, but after a certain wavelength infrared losses begin to dominate, thus producing a distinct minimum attenuation, which is characteristic for pure silica. Other materials—and in particular, other doped optical materials—may exhibit different frequency response.
As a consequence of FIG. 1A, and for a wavelength of injected light between 1540 to 1550 nanometers with an initial injection power level of one megawatt, FIG. 1B shows the theoretical limiting optical power transmission as a function of the length of fiber, indicating that as far as one-hundred kilometers from the laser source, that an output power of approximately fifty kilowatts (kW) is achievable. This level of output power is significant, and sufficient to enable a host of novel applications.
FIG. 1B is a theoretical construct. Prior to the work of the inventors, fiber optics have been limited to low power data communications applications and limited “power over fiber” demonstrations at very low power levels (on the order of milliwatts) over standard telecommunications fiber. At the other end of the spectrum, industrial cutting lasers, many powered by fiber lasers, have used a very short (typically less than ten meters in length) “process” fiber for transfer of the laser energy to a local cutting head adjacent the laser and in the same building.
The concept of very high power transfer over very long distances had not been investigated. The inventors, in the fall of 2007, began investigating the concept of using optical fiber to send tens of kilowatts of optical energy to an ice penetrating robotic system as a means of enabling a test of a planetary ice-cap penetrating science vehicle for the investigation of the polar ice caps of Mars as well as the planetary ice cap of the Jovian moon Europa. The concept was driven by a need to achieve thermal power levels at the robotic system that were similar to those that would be developed by a systems nuclear auxiliary power (SNAP) thermal reactor (on the order of several tens of kilowatts) without the use of nuclear power, as the likely testing grounds for the system would be Antarctica, where present treaties prohibit the use of nuclear power.
In early July 2010, the inventors conducted a high power, long range laser power transfer test that utilized a twenty-kilowatt infrared (1070 nanometer) fiber laser wherein power levels from zero to ten kilowatts were incrementally injected into a 1050-meter long coil of multi-mode, step index, pure silica core, fluoride doped cladded with polyimide coating (400 μm core, 440 μm cladding, 480 μm coating diameters). The fiber numerical aperture (NA) was 0.22.
FIG. 2 shows the results of that test, which compare favorably with the theoretical attenuation limits shown in FIG. 1A. The fiber was coiled into a one-meter diameter spool which was water cooled in a static flow bath, the temperature of which was monitored. The power was ramped up from one hundred watts to ten kilowatts over an approximately one-hour period. After five minutes at ten kilowatts, the peak temperature of the fiber was fifteen degrees Celsius above ambient as monitored using a forward looking infrared (FLIR) camera. This test pushed new boundaries in terms of the injected power levels sent through an optical fiber but also contradicted traditional thinking in the high power process laser industry that the power would have been completely dissipated by the large number (334) of bends to the fiber in the process of fabricating the coil.
With this as a background, we now discuss some important recent factors that enable practical implementation of the systems that will subsequently be described below. FIG. 3A shows a plot of raw industrial laser continuous output in kilowatts versus year for a single mode fiber laser. These were laboratory curiosities in the early 1990s. In 2009, however, an output level of ten kilowatts was achieved for a 1070 nm industrial fiber laser. It is important to note that with fiber lasers it is possible to combine several single mode lasers by injecting their individual beams into a multimode fiber. To date, multimode fiber lasers have achieved power levels of fifty to sixty kilowatts through one multimode fiber, operating over a short distance of process fiber (less than ten meters) between the laser and its output optics.
An equally important measure of progress is that of power density, expressed in megawatts per square centimeter (MW/cm2). FIG. 3B plots power density in MW/cm2 since 1994. A more practical means of understanding what this graph means is presented in FIG. 3C, in which the LOG of power density is plotted. This plot indicates that, at the current pace of development, which has been sustained since 1994, the power density will increase by an order of magnitude every six years.
Finally, FIG. 3D shows the theoretical power that can be transferred through a fiber optic carrier as a function of fiber core diameter (in microns) using optical power densities achieved in 2009. A three-hundred fifty micron core fiber is capable, today, of carrying a megawatt of optical power. FIG. 1B, as previously discussed, shows the output power that could be expected as a function of distance from the laser for a contiguous fiber.
The data presented in the figures referenced supra presage the possibility of sending enormous amounts of optical power over very long distances using very small diameter, lightweight fibers and converting that optical power to a more usable form of energy. Importantly, because the fiber is carrying the power, it will not be attenuated by the environment surrounding the fiber nor by a situation wherein the consumer of the power is not in visible line-of-sight of the source laser. This has profound implications on the development of many systems heretofore not considered possible.
FIG. 4 shows the very basic premise of the transfer of coherent high power laser radiation between a laser source 22 and a remote system 36 in which a base power source 20 (e.g., a nuclear power plant, a fossil fueled power plant, a large diesel generator, a very large array of solar cells, etc.) is used to provide electrical power to the high power fiber laser 22 via electrical conductors 24. Currently, the best fiber lasers are on the order of thirty-five percent power conversion efficiency (i.e., for every ten watts of raw electrical power, three-and-a-half watts of coherent laser radiation can be produced). Because of this, the laser 22 dissipates a substantial thermal heat load. To counteract this, a cooling system 26 and heat transfer system 28 is used to maintain thermal control at the laser. All of this infrastructure takes up volume, has significant mass, and consumes large amounts of power. It is therefore best located in some fixed ground facility or a large mobile facility (e.g., a ship). From the laser 22, a high energy process fiber 30 leads to a high power optical coupler 32, to which any number of devices can be connected. Traditionally, the only items to be connected to this category of multi-kilowatt laser are focusing optics for use in materials handling—e.g., cutting metal plates or fabric. Because of this, the length of the process fiber 30, 34 are generally quite short—on the order of five to ten meters.
The present invention relates to a system and apparatus that enables the transmission and effective end-use of very large amounts of optical power (e.g., kilowatts to tens of megawatts) over relatively long distances (e.g., from a kilometer to as much as one hundred kilometers or more) to fixed, movable, or mobile platforms operating on the ground, undersea, under ice, in the air, in space, and on other planets. The invention is usable in non-line-of-sight conditions, which allows it to directly bypass severe problems that have plagued efforts to utilize laser power beaming over large distances through the atmosphere, underwater, and over terrain where the receiver is not within view of the optical power source. The present invention permits first kilowatt and then ultimately multi-megawatt optical power injection and utilization over the length of a long deployed fiber.